Telecommunications networks are based in large part upon optical fiber. An electronic signal, similar to that conventionally transmitted over electrical lines in communications networks, is used to modulate the output of a laser at the transmitting site. The modulated optical signal is impressed upon an optical fiber that links the transmitting side to the receiving site. An optical detector has the receiving site detects the intensity of the optical signal, thereby regenerating the electronic signal at the transmitting site.
As the need for telecommunications bandwidth greatly increases, wavelength-division multiplexing (WDM) has been proposed as a way of multiplying the capacity of the existing optical fiber. As illustrated schematically in FIG. 1, a transmitter 10 at the transmitting site receives M electrical signals T.sub.1, T.sub.2, . . . T.sub.M corresponding to multiple telecommunications channels. Conventionally, the electrical signals, after proper conditioning, are used as power supply signals to M lasers 12. Importantly, the M lasers 12 each have distinctive wavelengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.m. For silica optical fiber, the wavelengths usually are chosen to fall in a band near 1300 nm or 1550 nm. For these wavelengths, the lasers 12 can be conveniently implemented as semiconductor laser diodes built of III-V compounds such as InP, its related materials, and the like. An optical multiplexer 14 receives the modulated outputs of the lasers 12 and combines them into a single multi-wavelength signal, which is then impressed upon an optical fiber 16 which enters a communications network 18.
At the receiving site, the operation is reversed in a receiver 20 receiving a multi-wavelength signal from an optical fiber 22 exiting the optical network 18. An optical demultiplexer 24 separates the optical signals of different optical carrier wavelengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.M and directs them to M optical detectors 26, usually implemented as photodetector diodes of InGaAs. The detectors 26 output separate electrical signals R.sub.1, R.sub.2, . . . R.sub.M for M communications channels.
In a point-to-point communications system, the two fibers 16,22 are connected together and there is a one-to-one correspondence between the transmit signal T.sub.i and the receive signal R.sub.i. However, in a multi-node WDM system, the optical signals can be switched within the communications network 18 and may be directed to different receivers 20. In addition, switching nodes within the network 18 may translate the signal from one wavelength to another one of those detected by the detectors 26.
For reasons of cost and reliability, much effort has been expended in integrating all or part of either the transmitter 10 or the receiver 20 onto a single integrated circuit chip. The transmitter 10 seems to present the more difficult integration. Zah has described an integrated laser array in U.S. Pat. No. 5,612,968. The multiplexer 14 can be a relatively simple star-coupler and can also be integrated on that chip. Zah et al. describe this further integration in "Wavelength accuracy and output power of multiwavelength DFB laser arrays with integrated star couplers and optical amplifiers," IEEE Photonics Technology Letters, vol. 8, no. 7, July 1996, pp. 864-866. However, Poguntke et al. have described in U.S. Pat. No. 5,351,262 that higher efficiencies can be obtained by using a grating in which the individual lasers or waveguides associated with them are disposed at positions corresponding to their diffraction wavelengths with respect to the grating. All the signals are then focused at an output channel. A similar use of geometry to determine the lasing wavelength is achieved with an array of waveguides instead of diffraction gratings, as disclosed by Zimgibl in U.S. Pat. No. 5,444,725.
Wavelength-division multiplexed networks with 8,16,32,64 . . . different wavelengths are now being considered. In order to minimize fiber loss, most systems will operate within the 1550 nm low-loss band for silica fiber. In large or complex networks, optical amplification is required, and the amplifier of choice is the erbium-doped fiber amplifier (EDFA), which provides gain over the wavelength band of 1530.about.1610 nm, although only a small fraction of that band is currently flat enough to allow signals to pass through a number of amplifiers in series. As a result, at the present time, the usable bandwidth is restricted to 1545.about.1560 nm.
The requirement to support a large number of wavelengths and the need to do this with wavelengths lying within a limited wavelength band imply that the lasing wavelength of each transmitter must be held with tight wavelength tolerance; any deviation from the designed wavelengths can seriously degrade the system performance.
Details both of the laser construction and of the laser operating conditions affect the lasing wavelength. The materials and dimensions of the layers comprising the laser structure and operating conditions of current and temperature all have to be maintained within tight limits. In the case of thermal effect, the temperature of the laser may be influenced by the laser drive current, by the proximity of other heat sources within the laser package (such as other laser elements on the same semiconductor chip), and by the external ambient temperature.
Accurate control of the wavelength of laser emission is one of the prime challenges of laser design for WDM systems.